PROJECT SUMMARY/ABSTRACT The overall objective of our research is to understand the detailed cellular and synaptic mechanisms underlying center and surround responses of retinal bipolar cells (BCs). Specifically, we will look at how neurons that mediate BC center and surround inputs in the outer retina (photoreceptors and horizontal cells (HCs)) interact with one another, and how adaptation, neuromodulators and the circadian clock regulate these cell-cell interactions and, consequently, BC receptive fields. We plan to use dual voltage clamp techniques to study cell-cell interactions at the photoreceptor, BC and HC levels, and use microelectrode voltage recording techniques to study receptive field properties of various types of cones, BCs and HCs. These two complementary approaches, along with the new knowledge and technical advances developed during the past years, will be applied to the salamander retina model system to study mechanisms underlying photoreceptor, HC and BC coupling and HC feedback/feedforward synaptic inputs to cones and BCs. These are common features of synaptic organization found in most vertebrate retinas. However, experiments involved in studying these features are technically difficult, if not impossible, to be carried out in many other vertebrates, including the mouse (thus issues to be addressed here complement, but do not overlap with our mouse grant). In this competing renewal application, we plan to continue our studies by focusing on 4 specific aims: (1) properties and modulation of electrical synapses between various pairs of photoreceptors (2 types of rods and 4 types of cones), (2) mechanisms underlying the sign-inverting, rod->cone transient signaling pathways in cones; (3) mechanisms underlying the depolarizing surround (feedback from HCs) responses in various types of cones; and (4) properties and modulation of HC and BC coupling and roles of cell-cell interaction in shaping BC and HC receptive fields. Results obtained will provide a detailed description of cell-cell interactions between neurons in the outer retina that are responsible for mediating the center and surround synaptic inputs in various types of BCs. Because of the similarities in outer retinal synaptic organizations between salamander and other vertebrates, knowledge obtained from this project will facilitate our understanding of how electrical/chemical synapses and BC receptive fields in mammalian and human retinas are organized, and provide important clues for developing animal models for various retinal diseases.